Such fans are used, among other purposes, to cool electrical cabinets. When such an electrical cabinet is opened, the fans are automatically switched off, but they continue to rotate for a while, and the risk therefore exists, that someone may bring his or her hand into contact with the sharp rotating blades on the rotor of such a fan. It is therefore important that, as soon as such a fan becomes currentless (for whatever reason), it is automatically decelerated very quickly, in order to minimize the risk of human injury.